I will speak from my experience as a man in his middle 60's, married decades, with a serious girlfriend and a few minimally successful light hookups(making out)

When you are married and even gently intimate that you would like to move in a more romantic direction with a female acquaintance, you run a big risk of scaring them off and thinking you are a creep. Many women are disgusted by a married man seeking other romantic partners. In an employment setting it can get you fired! In a friendship circle it can get you ostracized.

The problem is if you try to nip this reaction early by first letting the woman know you are in an "open" relationship, you often elicit the same response(they will think you are an immoral creep).

YES, YOU WILL RUIN POTENTIAL FRIENDSHIPS AS WELL AS YOUR REPUTATION!

So what to do?

-Go to poly meetups if available

-Try online dating(it has never worked for me)

-Seek women who are open to relationships with married men, if a woman is open to "cheating" she may well be open to ENM

-Slowly and cautiously flirt with women, do not escalate until they have given very strong cues that they are interested(this can be difficult as many women are not comfortable displaying sexually forward behavior). Unfortunately the woman may be waiting for you to make the first move in this direction! Each afraid to take the potential risk that rejection would bring.

Truthfully what you want is extremely hard for many of us to achieve - you want to engage in what our culture has determined to be inappropriate and creepy behavior without actually being seen as inappropriate and creepy!

Your experience with the shocked woman who later texted you shows that it can potentially work, but other women may react very differently. Generally women who send unsolicited texts would be good candidates to pursue romantically unless you know that is absolutely not what was intended.

Good luck - illicit affairs and prostitution have existed for centuries because NM is very difficult to find!